The authors report the pathological and clinical findings of a dissecting aneursym of the basilar artery in a 32 year old man up till then in good health, who died two months later. The pathological study showed bilateral lesions of osteo-malacia in the neighborhood of the pons due to hemo-dissection affecting the right vertebral artery just before it enters the dura-mater, then its intra-cranial portion, the basilar trunk and the first part of the right posterior cerebral artery. The originality of this case resides in the finding of segmental stenosing panarteritis of the same right vertebral artery in the segment which precedes dissection. On this occasion, the authors carried out a review of the literature and considered a few comments on the etiology, the pathology, clinical findings and classification of this unusual disease.